When storing or transporting radioactive substances, generated by nuclear power plants such as spent fuel having a high level of radioactivity and decay heat, the container used to hold this material must have a high radioactivity shielding capability, high seal performance, and have adequate cooling capabilities and structural strength. In general, concrete reinforced with steel rods or sheets has been used to fabricate such containers, but problems remain in the current implementations. One of the problems is the difference in the coefficients of thermal expansion between the concrete and the steel reinforcing materials.
Internally or externally reinforcing concrete using steel materials improves the strength of the container, but since the coefficient of thermal expansion of the steel materials is greater than that of the concrete, if the materials inside the container emit heat, cracks in the concrete could be generated to damage the container. Further, since the heat conductivity of concrete is lower than that of the metal, the additional problem being difficult to dispel heat generated inside the container to the outside exacerbates the above cited differences in their coefficients of thermal expansion even more to increase crack formation.
At this point, JP2000-162384A discloses a concrete cask which prevents the concrete cask container itself from reaching high temperatures.
As is shown in FIG. 4, the concrete cask 51 is comprised of concrete 55 formed to a cylindrical shape with a bottom, and an inner metal cylinder 56 on the inside circumference of container unit 53. Then, canister 52 is inserted into the container and the top opening is sealed by lid 54. A space 57 for the circulation of cooling air is disposed between the outside surface of canister 52 and container unit 53 and cooling air supply passages 58 and cooling air exhaust passages 59 are formed to connect thereto.
Thus, the structure enables cooling air to exhaust the heat from the inside of the container unit to the outside to thereby improve the durability and heat resistance of the container.
It is further disclosed to use a metal such as stainless steel, which has a coefficient of thermal expansion approximately equivalent to that of the concrete, to form the inner metal cylinder 56 as a reinforcing material for the concrete cask, as a means to minimize any damage to the cask and help it maintain its strength.
Further, JP2000-265435A discloses the use of polyethylene or other fiber sheets as a support frame instead of using steel as a reinforcing material to thereby simplify fabrication and reduce fabrication costs for concrete structures. According to this cited invention, a jacket would be formed from an outer sheet and inner sheet with a space disposed between them, and then the jacket would be immersed into the sea so as to introduce sea water into the space in the jacket, which would then be filled with concrete, which would displace the water, and be subsequently allowed to solidify to complete the structure.
However, just as is the case with the above cited JP2000-162384A, by simply forming passages for the flow of cooling air, when used to contain a high temperature heat emitting material with a high heat output, differences in the coefficients of thermal expansion of the materials could not be absorbed, and cracking would be inevitable. Further, not only does the use of stainless steel materials as disclosed make the fabrication more difficult, but it also entails much higher material costs.
On the other hand, while concrete structures according to the foregoing Patent Publication 2000-265435 would be suitable for holding low temperature materials, the use of polyethylene, or other fiber sheets for the support frame poses problems in the areas of both strength and heat resistance.